feat(D.2b): wire UiHost input + moveable windows (UiRoot window-drag + WantCapture gate)

- UiElement: add Draggable flag; left-drag on a draggable element repositions
  it as a floating window instead of starting a drag-drop sequence.
- UiRoot: add WantsMouse/WantsKeyboard properties (mirrors ImGui's WantCaptureMouse
  pattern); add FindDraggable helper; inject _windowDragTarget state machine into
  OnMouseDown/OnMouseMove/OnMouseUp so draggable windows track the pointer offset.
- UiNineSlicePanel: set Draggable=true so retail window frames are movable by default.
- GameWindow: OR _uiHost?.Root.WantsMouse|WantsKeyboard into the SilkMouseSource
  wantCaptureMouse/wantCaptureKeyboard delegates and the direct MouseMove gate so
  game actions (movement, world-pick) are suppressed while the pointer is over a
  retail window — no double-handling with the InputDispatcher.
- GameWindow: wire all Silk Mice/Keyboards to UiHost after construction so the
  UiRoot tree receives live input.
- Tests: 3 new UiRootInputTests covering WantsMouse hit-test, window-drag
  reposition, and non-draggable panel immobility.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

tests/AcDream.App.Tests/UI/UiRootInputTests.cs

@@ -0,0 +1,52 @@
+using System.Numerics;
+using AcDream.App.UI;
+
+namespace AcDream.App.Tests.UI;
+
+public class UiRootInputTests
+{
+    [Fact]
+    public void WantsMouse_TrueOverWidget_FalseOverEmptySpace()
+    {
+        var root = new UiRoot { Width = 800, Height = 600 };
+        var panel = new UiPanel { Left = 10, Top = 10, Width = 100, Height = 50 };
+        root.AddChild(panel);
+
+        root.OnMouseMove(50, 30);    // inside the panel
+        Assert.True(root.WantsMouse);
+
+        root.OnMouseMove(500, 400);  // empty space
+        Assert.False(root.WantsMouse);
+    }
+
+    [Fact]
+    public void WindowDrag_RepositionsDraggablePanel_StopsOnRelease()
+    {
+        var root = new UiRoot { Width = 800, Height = 600 };
+        var panel = new UiPanel { Left = 10, Top = 10, Width = 100, Height = 50, Draggable = true };
+        root.AddChild(panel);
+
+        root.OnMouseDown(UiMouseButton.Left, 20, 20); // grab at (10,10) into the panel
+        root.OnMouseMove(120, 90);                     // drag
+        Assert.Equal(110f, panel.Left);                // 120 - 10
+        Assert.Equal(80f, panel.Top);                  // 90 - 10
+
+        root.OnMouseUp(UiMouseButton.Left, 120, 90);
+        root.OnMouseMove(300, 300);                    // released — must not move
+        Assert.Equal(110f, panel.Left);
+        Assert.Equal(80f, panel.Top);
+    }
+
+    [Fact]
+    public void NonDraggablePanel_DoesNotMoveOnDrag()
+    {
+        var root = new UiRoot { Width = 800, Height = 600 };
+        var panel = new UiPanel { Left = 10, Top = 10, Width = 100, Height = 50 }; // Draggable defaults false
+        root.AddChild(panel);
+
+        root.OnMouseDown(UiMouseButton.Left, 20, 20);
+        root.OnMouseMove(120, 90);
+        Assert.Equal(10f, panel.Left);
+        Assert.Equal(10f, panel.Top);
+    }
+}